1. Field of the Invention
The present invention relates to amorphous semiconductor solar cells having p-i-n junctions, and more particularly to improvements of the photoelectric conversion efficiency thereof.
2. Description of the Background Art amorphous semiconductor solar cell has larger light absorbance, so that it has a large number of advantages such as a thin semiconductor layer, a simple manufacturing process, and a smaller energy requirement for manufacturing because of a low manufacturing temperature. In order to have satisfactory characteristics as a solar power cell, however, it still has some other problems to be solved such as further improvements in the photoelectric conversion efficiency and prevention of optical degradation (degradation of the photoelectric conversion efficiency due to light irradiation over a long time).
Accordingly, in view of the goal of obtaining improvements in the photoelectric conversion efficiency, an a-Si alloy solar cell using an a-Si alloy instead of amorphous silicon (a-Si) as an intrinsic (i type) semiconductor layer has been considered.
Referring to FIG. 1, one example of such an a-Si alloy solar cell according to the prior art is shown. On a conductive substrate 1, an n layer 2 of a-Si, an i layer 3 of amorphous silicon carbon (a-SiC) alloy, for example, and a p layer 4 of a-Si are sequentially provided. A transparent conductive film 5 is formed on the p layer 4. Since the a-SiC alloy i layer 3 has an energy bandgap larger than that of intrinsic a-Si, it can effectively photoelectrically convert light of short wave length to obtain a high open-circuit voltage V.sub.oc.
Referring to FIG. 2, curve 2A indicating the photoelectric conversion characteristics of a solar cell including an i layer of a-SiC alloy and curve 2B indicating the photoelectric conversion characteristics of a solar cell including an i layer of a-Si are shown for comparison. In this graph, the horizontal axis indicates open-circuit voltage V.sub.oc (V), and the vertical axis indicates short-circuit current Isc (mA/cm.sup.2). As seen from curves 2A and 2B, as compared to an a-Si solar cell, although the a-SiC alloy solar cell has an increased open-circuit voltage V.sub.oc because of effective use of short wave length light, it has a decreased short-circuit current Isc and decreased fill factor FF (obtained by dividing the maximum output by a product of the open-circuit voltage and the short-circuit current). That is, as a result, it can be seen that the a-SiC alloy solar cell does not have a photoelectric conversion efficiency significantly improved as compared to the a-Si solar cell.
On the other hand, if amorphous silicon germanium (a-SiGe) alloy is employed as the i layer 3 in the a-Si alloy solar cell of FIG. 1, since a-SiGe has a smaller bandgap as compared to a-Si, long wave length light can be effectively employed for photoelectric conversion. If p layer 4 is formed of a-SiC having a large bandgap in order to prevent optical absorption therein, a discontinuity of the bandgap is produced at the i-p interface. The discontinuity of the bandgap increases recombination of carriers at the i-p interface to reduce the open-circuit voltage V.sub.oc and the fill factor FF. Therefore in order to prevent the decrease of the open-circuit voltage V.sub.oc and the fill factor FF, as shown by the band structure in FIG. 3, it has been proposed to eliminate the discontinuity of the bandgap at the i-p interface by tapering the bandgap width of the i layer 3 in the vicinity of the i-p interface (see U.S. Pat. No. 4,816,32).
The film quality of an i layer of an a-Si alloy is, however, generally inferior to that of a-Si. Accordingly, in a solar cell with the entire thickness of the i-layer 3 formed of an a-Si alloy, the fill factor FF is reduced. Therefore, as shown in the bandgap structure of FIG. 4, it has been proposed to partially bring the film quality of i layer 3 close to the good film quality of the a-Si by varying the composition ratio of the alloy of the i layer 3 to vary the bandgap. That is, it has been proposed to improve the fill factor FF while maintaining the characteristics of the a-Si alloy material such as high sensitivity with respect to long wave length light and a decrease in voltage factor loss (see U.S. Pat. No. 4,816,082). In this case, it is reported that an internal electric field is produced by continuously varying the bandgap of the i layer to implement a gradient, which facilitates movement of carriers.
Furthermore, a development project of tandem type solar cells in which a plurality of solar cells employing a-Si alloy materials are stacked up as solar cells capable of implementing high efficiency is in progress.
As described above, although solar cells employing a- alloy materials have been improved recently, further improvements of photoelectric conversion efficiency for obtaining solar cells with higher efficiency are still required.